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https://issues.apache.org/jira/browse/SOLR-12259?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16662584#comment-16662584
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Steve Rowe commented on SOLR-12259:
-----------------------------------
In addition to the issues mentioned on SOLR-11127 (upgrading .system collection
schema from Trie to Points fields), Solr needs somehow to be able to re-index
the .system collection on major version upgrade, because of the unsupported
index version N-2 problem.
> Robustly upgrade indexes
> ------------------------
>
> Key: SOLR-12259
> URL: https://issues.apache.org/jira/browse/SOLR-12259
> Project: Solr
> Issue Type: Improvement
> Security Level: Public(Default Security Level. Issues are Public)
> Reporter: Erick Erickson
> Assignee: Erick Erickson
> Priority: Major
>
> The general problem statement is that the current upgrade path is trappy and
> cumbersome. It would be a great help "in the field" to make the upgrade
> process less painful.
> Additionally one of the most common things users want to do is enable
> docValues, but currently they often have to re-index.
> Issues:
> 1> if I upgrade from 5x to 6x and then 7x, theres no guarantee that when I go
> to 7x all the segments have been rewritten in 6x format. Say I have a segment
> at max size that has no deletions. It'll never be rewritten until it has
> deleted docs. And perhaps 50% deleted docs currently.
> 2> IndexUpgraderTool explicitly does a forcemerge to 1 segment, which is bad.
> 3> in a large distributed system, running IndexUpgraderTool on all the nodes
> is cumbersome even if <2> is acceptable.
> 4> Users who realize specifying docValues on a field would be A Good Thing
> have to re-index. We have UninvertDocValuesMergePolicyFactory. Wouldn't it be
> nice to be able to have this done all at once without forceMerging to one
> segment.
> Proposal:
> Somehow avoid the above. Currently LUCENE-7976 is a start in that direction.
> It will make TMP respect max segments size so can avoid forceMerges that
> result in one segment. What it does _not_ do is rewrite segments with zero
> (or a small percentage) deleted documents.
> So it doesn't seem like a huge stretch to be able to specify to TMP the
> option to rewrite segments that have no deleted documents. Perhaps a new
> parameter to optimize?
> This would likely require another change to TMP or whatever.
> So upgrading to a new solr would look like
> 1> install the new Solr
> 2> execute
> "http://node:port/solr/collection_or_core/update?optimize=true&upgradeAllSegments=true";
> What's not clear to me is whether we'd require
> UninvertDocValuesMergePolicyFactory to be specified and wrap TMP or not.
> Anyway, let's discuss. I'll create yet another LUCENE JIRA for TMP do rewrite
> all segments that I'll link.
> I'll also link several other JIRAs in here, they're coalescing.
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